The present invention generally relates to the field of signal equalization circuits in which a selected signal undergoes selective relative emphasis of various frequency subbands which comprise the selected signal. More specifically the present invention relates to an improved audio signal equalization circuit in which an audio signal selector is not only used to determine which audio signal will be subjected to a desired predetermined signal equalization, but the signal selector also simultaneously and automatically selects one of several pluralities (sets) of stored control signals to implement the desired equalization while simultaneously selecting the audio signal which is to undergo equalization.
In general, audio equalizers are available which provide for the separate manual adjustment of the relative emphasis of a plurality of contiguous frequency subbands of a selected audio signal so that the audio frequency response of an entertainment device can be operator adjusted so as to suit the operator's personal preference. One such audio equalizer which provides this function through the use of a plurality of stored control signals which are stored in a random access memory (RAM) is illustrated in U.S. Pat. No. 4,285,065, entitled "Radio with Audio Graphic Equalizer", which is assigned to the same assignee as the present invention. That patent describes a AM radio having an integral audio equalizer in which stored digital equalizer control signals can be manually adjusted in their digital magnitude so as to provide the radio with an operator controllable audio equalization for the output audio signal of the radio.
A radio with an integral audio equalizer, such as that shown in the '065 patent, can be utilized to provide any desired audio equalization for its audio output. However, the radio in the prior patent appears to contemplate providing the same audio equalization for any audio signal which may be selected for equalization by any audio signal program selector, such as an AM or FM radio switch or a radio mode versus tape player mode switch, unless the equalization is additionally adjusted for each selected audio signal after the selection of the audio signal. The '065 patent does mention selectively implementing an equalizer "defeat" mode in which no relative audio emphasis is provided for any of the subbands in response to actuation of a equalizer defeat switch, but the manner of carrying this out is not described. Also it is clear that actuation of the equalizer defeat switch does not result in selecting a different audio signal for equalization, but merely implements an effectively different equalization for the same audio signal.
Some prior audio entertainment devices have recognized that different predetermined equalization characteristics may be desired for different selected audio entertainment signals. Typically these devices have implemented different signal equalizations for different selected audio signals by the selective coupling of fixed equalization circuits into the selected audio signal path in accordance with what audio program signal has been selected. In other words, if an AM radio mode of operation were selected, a first fixed equalization circuit, such as a 10 kHz whistle notch filter, would be switched into the audio signal path whereas if a record player mode of operation were selected a second equalization circuit, such as a low frequency rumble filter, would be switched into the audio signal path. These types of prior signal equalization selector assemblies have heretofore only involved the use of fixed equalization networks, thus depriving the operator of the entertainment device, unless additional equalization circuits are used, of the ability to selectively adjust the amount of equalization to his own personal taste for each of the different types of audio program signals which may be selected.